DE 10 2008 019 699 A1 discloses e.g. a belt test stand for motor vehicles, which includes a belt unit having a first drum and a second drum. One of these drums is driven and the other one is carried along via the endless belt. A hydraulic cylinder is arranged between the drums and increases or decreases the distance between the driving drum and the idler drum depending on the moving direction of the piston so as to correspondingly increase or decrease the pretension of the endless belt. It is also shown that one of the drums is fixedly arranged in the belt running direction and the other drum is movably mounted. In addition, a support is shown the top side of which is provided between the drums. The top side of the support is arranged here in alignment with a tangential connecting line of the top side of the drums.
DE 197 02 421C2 describes a test stand for determining forces and moments acting on a motor vehicle or on components thereof, wherein a wheel belt unit is mounted on a weighbridge. The threshold support also rests on the weighbridge. However, a recess in the non-weighed base plate is chosen to be relatively great, and therefore the area for generating error forces is relatively large.
A drawback of the above mentioned prior art is that the belt test stand area exposed to the air current is relatively large, and therefore error forces are measured by the test stand.
In order to optimize the aerodynamic properties of a motor vehicle, the motor vehicles are exposed to real wind conditions in what is called a wind tunnel. It is here of utmost significance that all current conditions around the car body and at the underbody are reproduced as realistically as possible.
This is why the vehicle is not placed on otherwise common roller type test stands but on what is called flat belt units (belt test stands) since the latter permit a level insertion in the base plate.
What is called a center conveyor belt can be inserted here in the center of the test apparatus and is usually operated at a same speed as that of the air current. As a result, realistic pressure conditions occur at the underbody of the motor vehicle. During an incoming fluid flow, the vehicle forces must be measured via “X” (from the front) and via “Y” (from the side) and the lifting force “Z”. The lifting force is understood to mean here the difference between the normal weight force without an incoming flow and the weight force with the incoming flow. In accordance with the three forces in the X-, Y- and Z-directions, it is also possible to suitably determine three moments in the X-, Y- and Z-directions by including the lever arms, and therefore such an apparatus is referred to as a 6-component balance.
Since the vehicle would be pushed off the belt units by the X-forces occurring, it is necessary to hold the vehicle via beam supports (so-called rocker panels). Here, the four wheel belt units and the four threshold supports are mounted on the weighing frame. All the other parts of the platform are connected to the non-weighted test stand design.
In a model case, the belt contact area which is visible from above is as large as the tire shuffle of the respective wheels of the motor vehicle to be tested. Of course, this can only be achieved theoretically. In practice, the wheel belt units are markedly greater than the tire shuffle. As a result, parts of the wheel belt unit and thus of the wind tunnel balance are exposed to the resulting flow conditions at and/or under the vehicle. In accordance with the resulting pressure condition and the exposed area, error forces are formed and add up to the actual lifting force and/or downforce.